2019-05-21 09:44:01

by Yang Shi

Subject: [v3 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned

The commit 9092c71bb724 ("mm: use sc->priority for slab shrink targets")
has broken up the relationship between sc->nr_scanned and slab pressure.
The sc->nr_scanned can't double slab pressure anymore. So, it sounds no
sense to still keep sc->nr_scanned inc'ed. Actually, it would prevent
from adding pressure on slab shrink since excessive sc->nr_scanned would
prevent from scan->priority raise.

The bonnie test doesn't show this would change the behavior of
slab shrinkers.

w/ w/o
/sec %CP /sec %CP
Sequential delete: 3960.6 94.6 3997.6 96.2
Random delete: 2518 63.8 2561.6 64.6

The slight increase of "/sec" without the patch would be caused by the
slight increase of CPU usage.

Cc: Josef Bacik <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Signed-off-by: Yang Shi <[email protected]>
---
mm/vmscan.c | 5 -----
1 file changed, 5 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 7acd0af..b65bc50 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1137,11 +1137,6 @@ static unsigned long shrink_page_list(struct list_head *page_list,
if (!sc->may_unmap && page_mapped(page))
goto keep_locked;

- /* Double the slab pressure for mapped and swapcache pages */
- if ((page_mapped(page) || PageSwapCache(page)) &&
- !(PageAnon(page) && !PageSwapBacked(page)))
- sc->nr_scanned++;
-
may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));

--
1.8.3.1

2019-05-21 15:48:31

by Johannes Weiner

Subject: Re: [v3 PATCH 1/2] mm: vmscan: remove double slab pressure by inc'ing sc->nr_scanned

On Tue, May 21, 2019 at 05:40:41PM +0800, Yang Shi wrote:
> The commit 9092c71bb724 ("mm: use sc->priority for slab shrink targets")
> has broken up the relationship between sc->nr_scanned and slab pressure.
> The sc->nr_scanned can't double slab pressure anymore. So, it sounds no
> sense to still keep sc->nr_scanned inc'ed. Actually, it would prevent
> from adding pressure on slab shrink since excessive sc->nr_scanned would
> prevent from scan->priority raise.
>
> The bonnie test doesn't show this would change the behavior of
> slab shrinkers.
>
> w/ w/o
> /sec %CP /sec %CP
> Sequential delete: 3960.6 94.6 3997.6 96.2
> Random delete: 2518 63.8 2561.6 64.6
>
> The slight increase of "/sec" without the patch would be caused by the
> slight increase of CPU usage.
>
> Cc: Josef Bacik <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: Johannes Weiner <[email protected]>
> Signed-off-by: Yang Shi <[email protected]>

Acked-by: Johannes Weiner <[email protected]>